Seismic prospecting as practiced for the purpose of hydrocarbon (and mineral) exploration is primarily interested in backscattered, or reflected, waves from the earth's subsurface. The method of active seismic exploration uses seismic sources, at or near the surface of the earth (in the case of land seismic prospecting) or the sea (in the case of marine seismic prospecting), to excite seismic waves that propagate down into the earth. The waves encounter impedance changes in the earth and are reflected or scattered. Some of the waves propagate back to the surface where sensors (or receivers) record their amplitudes and times of arrival. The recorded data are then used to determine structure and physical properties of the earth's subsurface.
A major challenge in the modern practice of active seismic exploration is limits in the range of frequencies that can be implemented. The larger the frequency range that can be implemented, the more information that is obtained and can be used for improved determination of structure and physical properties of the earth's subsurface. For example, frequencies below the range presently implemented by modern practice have been long desired for improved seismic inversion. The primary obstacle has been the lack of practical seismic sources that can excite seismic waves into the earth at these frequencies. In spite of many decades of efforts to resolve this problem, there remains considerable room for advancements in seismic source concepts and technologies that can improve active seismic exploration method at theses frequencies.
Many types of seismic sources are commonly used in seismic prospecting. Some source types consist of charges of gun powder, dynamite, or other chemical explosives that releases short bursts of energy. In the marine setting, air-guns are often used to release a volume of compressed air into the water, which forces the surrounding water and excites a seismic wave. Other source types use mechanical means to impart an oscillatory force with controlled characteristics, such as force level and frequency, on the earth or in the water.
In all cases, seismic sources have characteristics and limitations that govern their utility for seismic prospecting. Among the more important characteristics is the force output a source generates, and how the force output varies at different frequencies. Most sources can impart a force sufficiently large over some certain range of frequencies to excite seismic waves at those frequencies that can travel through the earth, reflect or scatter off impedance changes in a region of prospecting interest in the earth's subsurface, and travel back to the surface with sufficient amplitude to be measured by sensors (or receivers). That certain range of frequencies can be referred to as the seismic bandwidth. However, outside that certain range of frequencies, there is either no force output, or the force output is too small to excite a seismic wave with sufficient amplitude to be measured by the sensors (or receivers).
A source frequently used in land seismic acquisition is the seismic vibrator. The modern seismic vibrator rests a baseplate on the ground, and uses hydraulic actuators connected to the baseplate and an inertial mass to drive an oscillatory displacement between them. FIG. 1 is a schematic representation of a seismic vibrator showing a baseplate 101, a hydraulic actuator 102, and an inertial mass 103. The hydraulic actuator contains a cylinder 106 that houses a piston 104. The piston is rigidly connected to the baseplate 101. A hydraulic system forces hydraulic fluid through valves 105 to the cylinder thereby modulating the displacement of the mass relative to the baseplate. Considering the case where the baseplate is relatively immobile (i.e., the “clamped force” case) the force to accelerate the inertial mass in an oscillatory motion described byx=d cos(2πft),  [1]where x is the motion of the mass, d is the maximum displacement of the mass from a center position, f is the frequency of motion, and t is the independent variable of time; is given by the product of mass and acceleration (that is, second derivative of motion),F=m{umlaut over (x)}=−md4π2f2 cos(2πft),  [2]where F is the force and m is the mass of the inertial mass. [In reality, the baseplate does move somewhat; those skilled in the art can compute the force knowing the accelerations and masses of the inertial mass and baseplate. The clamped force description is given here for simplicity.] The reaction force generated by acceleration of the inertial mass is transferred to the baseplate and imparts an oscillating force to the ground. The oscillating ground force excites seismic waves.
The oscillating force imparted on the ground and how it varies over time can be called a sweep or ground force sweep or a controlled sweep. In practice, a sweep is usually more complicated than a simple cosine function, but still oscillatory in nature, and approximately cosine locally in time. Sweeps can typically be a few seconds long or longer, and have variation in the frequency and magnitude of oscillation of the ground force (the magnitude of oscillation of the ground force sometimes called the magnitude or envelope of the ground force, or just the ground force) over the duration of the sweep. An operator, or sweep control system, can independently change the frequency and the magnitude of ground force. For example, an operator or sweep control system may change the frequency while keeping the ground force unchanged during some portion of the sweep, or may change the ground force while keeping the frequency unchanged, or a combination of changing frequency while changing ground force, or keeping ground force unchanged while keeping frequency unchanged. The sweep so implemented can be referred to as a “controlled sweep”. Oftentimes, the sweep is pre-programmed, and the sweep control system will implement the sweep upon command by the operator or by radio control. There are many models of land vibrators used commercially in seismic prospecting today. Many popular models are rated to provide up to 275 kN of ground force at frequencies between 5 Hz and 250 Hz. Some of the largest models are rated to provide up to 400 kN of ground force at frequencies between 5 Hz and 250 Hz. Forces in the range of about 275 kN to 400 kN may be referred to as “large forces”.
Another important feature of seismic vibrators for commercial applications in seismic prospecting is mobility. Modern seismic vibrators can move between locations, so as to be able to do one or more controlled sweeps at a location, then move to another nearby location and do one or more controlled sweeps, and so on throughout a seismic survey area. A modern seismic acquisition program will often excite seismic waves at many thousand locations, or source stations. A source that moves quickly and efficiently between source stations is more practical.
An important limitation of modern seismic vibrators used in seismic prospecting is the force output at low frequencies. The devices can typically provide the maximum rated force only to frequencies down to about 5 Hz. At this point, the hydraulic actuators reach their maximum displacement capability, typically less than about 5 cm from a center position, i.e., a total stroke less than about 10 cm. Because the displacement is at the maximum, and cannot be further increased as frequencies are lowered, the force output falls with the square of the frequency. Hence the seismic vibrator rated to provide a force of 400 kN down to 5 Hz may only be able to provide a force of 100 kN at 2.5 Hz, a force of 25 kN at 1.25 Hz, and a force of 16 kN at 1 Hz. The small forces may not be adequate for seismic prospecting at these lower frequencies. One way to compensate for smaller forces is to sweep for much longer durations. However, longer duration sweeps increase the time required to conduct a seismic survey, and correspondingly increases the cost. A solution that is more effective and less costly may be to increase the force output capability of seismic vibrators at low frequencies.
In addition to seismic vibrators, it is also widely known, at least empirically, that seismic sources of all types used commercially in the seismic prospecting industry, including air guns used in marine seismic prospecting, tend to follow similar trends, substantially losing force output as the frequency falls below about 5 Hz. While it is generally accepted that seismic prospecting above 5 Hz is well practiced and established, frequencies lower than 5 Hz become more difficult, and frequencies near 1 Hz are not successfully achieved in modern seismic prospecting. Frequencies below 5 Hz and including 1 Hz can be referred to as the “low frequencies.”
The potential value low frequencies may contribute in seismic prospecting has been known for many decades. The seismic prospecting industry can greatly benefit from improvements in low frequency capabilities. One way to improve seismic prospecting at low frequencies is to develop a seismic source that can generate large forces at low frequencies. An example would be a seismic source that can produce forces in the range of about 275 kN to 400 kN at frequencies below 5 Hz and including 1 Hz. The present invention satisfies this need.